A condensable gas, such as SF.sub.6 gas, is confined as an insulating gas in a gas insulator such as a gas insulation switching apparatus installed in a power plant, and it is necessary to open the gas insulator when the power plant is inspected. To recover the condensable gas such as SF.sub.6 gas confined in the apparatus without discharging it into the air at the time of inspection has been demanded in view of economics or prevention of global warming.
In order to meet this demand, Japanese Published and Examined Patent Application No. Hei. 5-78718 proposed a gas replacing and recovering apparatus for recovering a condensable gas used as an insulating gas, such as SF.sub.6. FIG. 13 is a system diagram showing the gas replacing and recovering apparatus. In the figure, reference numeral 1301 designates an insulating gas liquefying, replacing and recovering apparatus comprising a carburetor 1313, a dryer 1307, a gas compressor 1308, an oil separator 1309, an air cooler 1310, an exhaust pipe, and end connections 1305 and 1306. The end connections 1305 and 1306 are members for connecting the insulating gas liquefying, replacing and recovering apparatus 1301 with a container 1302 described later through an upper port 1303 and a lower port 1304, respectively.
The container 1302 is, for example, an outer container for a gas insulation switching apparatus employed in a power plant.
The insulating gas liquefying, replacing and recovering apparatus 1301 is used for recovering the condensable gas in the container without discharging it to the air when the condensable gas is introduced into the container or when the container is opened. Hereinafter, a description is given of a method for replacing air in the container 1302 with a desired amount of SF.sub.6 gas and recovering an excess of SF.sub.6 using the insulating gas liquefying, replacing and recovering apparatus 1301.
Initially, liquefied SF.sub.6 is introduced from an SF.sub.6 bomb (not shown) into the insulating gas liquefying, replacing and recovering apparatus 1301, and the liquefied SF.sub.6 is vaporized and decompressed by the carburetor 1313. Then, the vaporized SF.sub.6, namely, SF.sub.6 gas is introduced through the end connection 1306 and the lower port 1304 into the container 1302.
When the SF.sub.6 gas is introduced into the container 1302, it mixes with air which is an incondensable gas and has already been existing in the container, and a mixed gas so produced is introduced through the upper port 1303 and the end connection 1305 into the insulating gas liquefying, replacing and recovering apparatus 1301.
The mixed gas introduced into the apparatus 1301 is completely dried by the dryer 1307 and, thereafter, it is processed through the gas compressor 1308, the oil separator 1309 and the air cooler 1310, to reach the cooling separator 1311.
When the mixed gas is cooled in the cooling separator 1311, the SF.sub.6 gas liquefies before the air. So, the liquefied SF.sub.6 is collected, and the mixed gas comprising the residual air and the unliquefied SF.sub.6 gas is discharged through the exhaust pipe 1312 to the air. The liquefied SF.sub.6 is again vaporized and sent to the container 1302. This work is repeated until the container 1302 is filled with a desired amount of SF.sub.6 gas. In the case of simply collecting the SF.sub.6 gas, the liquefied SF.sub.6 is collected.
The insulating gas liquefying, replacing and recovering apparatus 1301 performs liquefaction and replacement of the insulating gas as described above and is able to recover the insulating gas according to the application. In the apparatus 1301, however, when a mixed gas comprising an incondensable gas such as air and a condensable gas such as SF.sub.6 gas is cooled, the condensable gas equivalent to the vapor pressure remains as it is in the incondensable gas under the cooling temperature. When this incondensable gas is discharged to the air as exhaust gas, the condensable gas is also discharged to the air, resulting in economical and environmental problems.
If an insulating gas to be processed comprises a pure condensable gas alone, since only a very small amount of the condensable gas exists in the incondensable gas when discharging the incondensable gas to the air according to the above-mentioned process, the absolute quantity of the condensable gas discharged to the air is very small and this does not lead to a serious problem. However, as in the case where an insulating gas to be processed is a mixed gas from the beginning, when processing a mixed gas in which the percentage of an incondensable gas is larger than the percentage of a condensable gas, the quantity of the condensable gas discharged to the air increases and, therefore, this problem is not negligible.
With a view to recovering a condensable gas without discharging it to the air, Japanese Published Patent Application No. Hei. 9-285719 proposed an SF.sub.6 gas recovery and reproduction apparatus and a movable recovery and reproduction apparatus, wherein a very small amount of incondensable gas, which is generated inside a gas insulator or mixed during the process of gas replacement, is completely adsorbed into an adsorbent to realize recovery of a condensable gas containing no incondensable gas.
The technique disclosed in this prior art will be described with reference to FIG. 14. As shown in FIG. 14, SF.sub.6 gas including a very small amount of incondensable gas is compressed by a compressor 1401 and then introduced through a first air adsorber 1402 into a liquefier 1404. The first air adsorber 1402 contains a synthetic zeolite 1403, and the incondensable gas included in the SF.sub.6 gas, such as nitrogen gas and oxygen gas, is adsorbed into the synthetic zeolite 1403, where by the incondensable gas is removed and pure SF.sub.6 is recovered.
However, the above-described technique has the following drawback. Although this technique is effective when the absolute quantity of the incondensable gas is very small, it cannot be applied to a mixed gas in which the percentage of an incondensable gas is apparently larger than that of SF.sub.6 gas.